In data manipulation systems handling packets of data arranged in frames (a packet, or burst, of data including certain framing, or "housekeeping" bits is defined as a "frame"; data sent in the from of frames is termed "framed data"), there is often a requirement to send the framed data from one set of queues at one location to a second set at another location. For example, in a network of a type described in copending application (1), supra, for "FDDI Controller Having Flexible Buffer Management", incorporated herein by reference, there is a network controller comprising a bus master architecture wherein queues of framed data are transferred from buffers established in a system memory to corresponding regions formed by logical FIFOs in an output buffer for transmission to a network. Between the system memory and the output buffer is a physical FIFO having a predetermined storage capacity, e.g., 32 bytes, for storing the framed data temporarily en route to the output buffer. One purpose of the physical FIFO is to provide buffering of data that is required as a result of differences in the clock rates of system and network clocks.
The queues of data handled by the FDDI network controller are transferred from buffer regions in the system memory to the output buffer, one frame at a time, in response to a request for transmission to the network upon receipt of a "token" (in FDDI, the network is composed of an optical medium) and data available for transmission. The queues of data are stored in corresponding queues formed by "logical FIFOs" in the output buffer.
Data is transferred in order of priority beginning with the highest until no additional data having the same priority is available for transmission or the unexpired token holding time (THT) during receipt of the token have become less than a threshold value for that priority. Details on this protocol are given in copendinq application (3), supra, for "Method of and System for Implementing Multiple Levels of Asynchronous Priority in FDDI Networks", incorporated herein by reference.
Following each transfer of framed data from the system memory, through the physical FIFO, to the output buffer, a decision is made either to transfer additional data having the same priority to the physical FIFO to thereafter be transferred to the same output buffer queue, or to transfer data having a different priority to the physical FIFO, if any additional data is available, to thereafter be sent to another output buffer queue.
However, a condition termed "locking-up" of the physical FIFO will tend to occur, making further transfer of data through the FIFO impossible, unless the FIFO is "flushed". Locking-up occurs when a queue in the output buffer is "almost full", that is, when the amount of storage remaining available in the logical FIFO containing that queue, is less than the storage capacity of the physical FIFO. When this occurs, the physical FIFO does not completely empty of data having the current priority as data having a different level of priority begins to be transferred to it from the system memory. Locking-up occurs because the residual data stored in the physical FIFO cannot be transferred to the new output buffer queue because the new queue has a level of priority different from that of the residual data in the FIFO. This phenomenon must be prevented from occurring, and at the same time, any available data must be transferred to the physical FIFO immediately when the current queue is not "almost full", to optimize the operating speed of the network.